Conventional glass ionomer cements generally contain a powder component containing aluminosilicate, and a liquid component usually containing an aqueous mixture containing a polymer comprising acidic groups such as polyacrylic acid, polymaleic acid, polyitaconic acid, or a copolymer of at least two of these acids, cf. “New Aspects of the Setting of Glass-ionomer Cements,” Wasson et al., Journal of Dental Research; Vol. 72, No. 2, February, 1993; pages 481-483. The most common polymers comprising acidic groups are derived from polyacrylic acid or copolymers of acrylic and itaconic acid (S. Crisp), acrylic acid and maleic acid.
In glass ionomer cements, the primary reactions which cause the glass ionomer cement to harden is crosslinking based on ionic forces between metal ions released from the glass and the polymer comprising acidic groups. Moreover, the acids of the glass ionomer cement partially dilute metal cations from the glass structure during setting so that ionic carboxylates of metal cations may be formed during the setting process.
Glass ionomers used as dental restoratives have advantages over conventional resin containing composites for several reasons. For example, glass ionomers are tolerant to application on wet surfaces, have low shrinkage and are self-adhesive. Since glass ionomers contain polymers rather than monomers, there is no risk of acrylic monomers leaching out, which can lead to sensitization and allergic reactions. Furthermore, glass ionomers bond chemically to dental hard tissues, and may also provide a beneficial level of fluoride release, which helps to prevent recurrent caries. Accordingly, ionomer cements are widely used in the dental field for filling of a cavity, cementing of crowns, inlays, bridges, or orthodontic bands, lining of a cavity, sealing of a root canal, core construction, and preventive sealing.
A key weakness of commercial glass ionomers, however, is their low flexural strength manifesting itself as an undesirable brittleness, which may lead to fracture at the edges of a restoration and, in the worst case, to bulk fracture of a restoration. Therefore, the restorative application of ionomer cements in posterior teeth is usually limited to non-stress bearing areas. ionomer cement materials continue to have significant limitations for use in permanent posterior restorations, particularly with regard to large restorations.
In order to improve the mechanical properties especially flexural strength and fracture toughness, numerous investigation were carried out, such as the use of amino acid modified polymers (Z. Ouyang, S. K. Sneckberger, E. C. Kao, B. M. Culbertson, P. W. Jagodzinski, Appl. Spectros 53 (1999) 297-301; B. M. Culbertson, D. Xie, A. Thakur, J. Macromol. Sci. Pure Appl. Chem. A 36 (1999) 681-96), application of water soluble copolymers using poly(N-vinylpyrrolidone) (D. Xie, B. M. Culbertson, G. J. Wang, J. Macromol. Sci. Pure Appl. Chem. A 35 (1998) 54761), use of polyacids with narrow molecular weight distribution (DE 100 58 829) and branched polyacids (DE 100 58 830). Further polyacids having a limited molecular mass ranging from 20,000 to 50,000 Da (EP 0 797 975) and 1,000 to 50,000 Da (WO 02/41845) were proposed. A further approach was the application of spherical ionomer particles (WO 00/05182).
Resin-modified glass-ionomer cements were introduced with an aim of overcoming the problems associated with the tendency towards brittle fracture of conventional glass-ionomer, while still retaining advantages such as fluoride release and adhesion (EP 0323120, U.S. Pat. No. 4,872,936 and U.S. Pat. No. 5,154,762). Accordingly, it was suggested to replace some of the water in a conventional glass-ionomer cement with a hydrophilic monomer or to modify the polymeric acid so that some of the acid groups were replaced with polymerizable moieties, so that the polymeric acid could also take part in a polymerization reaction.
Moreover, in order to address the problem of improving the mechanical properties of ionomer cement materials, U.S. Pat. No. 5,369,142 suggests the use of a specific acidic component, namely copolymers of acryloyl or methacryloyl derivatives of amino acids with acrylic acid or methacrylic acid. WO-A 02/062861 discloses polymer compositions for use in glass ionomer dental restoratives having improved resistance to bending and resistance to twisting, whereby the polymers are formed from at least two specific polymers. WO-A 03/061606 discloses ionomer cements containing amino acids improving the mechanical properties.
US 2002/0010227 discloses light-curable acid containing polymers in aqueous solution, which are obtainable by reacting polymers having reactive carboxylic acid groups with a methacrylated oxazoline or oxazine. WO03/011232 discloses resin-modified glass ionomer cements comprising a polymer having a plurality of acidic moieties and a plurality of polymerizable vinyl groups. The introduction of polymerizable moieties into a polyacrylic acid according to the prior art as set out in US 2002/0010227 or WO03/011232 means that water solubility of the polyacrylic acid deteriorates, which is not desirable in view of the viscosity and handling properties of a dental cement. Moreover, in case of WO03/011232, the leaching of HEMA from the cured composition and volume expansion of the cured cement represent major problems for an application in the dental field.